P-channel demos device

ABSTRACT

A p-channel drain extended metal oxide semiconductor (DEPMOS) device includes a doped surface layer at least one nwell finger defining an nwell length and width direction within the doped surface layer. A first pwell is on one side of the nwell finger including a p+source and a second pwell is on an opposite side of the nwell finger including a p+drain. A gate stack defines a channel region of the nwell finger between the source and drain. A field dielectric layer is on a portion of the doped surface layer defining active area boundaries including a first active area having a first active area boundary including a first active area boundary along the width direction (WD boundary). The nwell finger includes a reduced doping finger edge region over a portion of the WD boundary.

FIELD

Disclosed embodiments relate to p-channel drain extended metal oxidesemiconductor (DEPMOS) devices.

BACKGROUND

Power semiconductor devices can be fabricated using n- or p-channelDEMOS structures. DEPMOS devices extend the p+drain of the device byadding a p-type drain drift region between the drain and the channelregion of the device, trapping the majority of the electric field inthis drift region instead of the channel region, therefore containinghot carrier effects to the drift region, instead of the channel regiontherefore increasing hot carrier reliability. The DEMOS device can havea symmetric drain structure or an asymmetric drain structure.

SUMMARY

This Summary is provided to introduce a brief selection of disclosedconcepts in a simplified form that are further described below in theDetailed Description including the drawings provided. This Summary isnot intended to limit the claimed subject matter's scope.

Disclosed embodiments recognize for conventional p-channel drainextended metal oxide semiconductor (DEPMOS) devices having fingers thenwell finger (functioning as a n-doped body) can have the n-type dopingpiled up at the semiconductor surface at the finger ends under the fielddielectric layer. The n-type doping pile up is particularly a problem inthe case the field dielectric layer is a Local Oxidation of Silicon(LOCOS) oxide and the region is the birds beak area. This higher n-typedopant concentration has been found to result in increased impactionization DEPMOS device leakage at ON-state with high back gate biaslevels causing increased transient leakage and parametric shifts (e.g.,in ON-resistance (Ron)).

Disclosed nwell finger designs have reduced doping finger edge regionsincluding over an active area/field dielectric boundary to reduce impactionization leakage at the finger-ends. A p-doped layer can added(typically by ion implantation) at the finger end inside the nwell edgeclose to the active region boundary with the field dielectric. Anotherembodiment has a gap in the nwell implant doping through masking thenwell implant at the finger end inside the nwell edge close to theactive region boundary with the field dielectric.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the accompanying drawings, which are notnecessarily drawn to scale, wherein:

FIG. 1 is top view depiction of an integrated circuit (IC) including anexample DEPMOS device having a disclosed reduced doping finger edgeregion within the nwell finger including over an active area/fielddielectric boundary, according to an example embodiment.

FIG. 2A is a cross sectional depiction of the DEPMOS device shown inFIG. 1 cut in the nwell length direction around the active area/fielddielectric boundary along the nwell width direction.

FIG. 2B is a cross sectional depiction of the DEPMOS device shown inFIG. 1 cut in the nwell width direction around the active area/fielddielectric boundary along the nwell width direction.

FIG. 3 is a depiction of the DEPMOS device shown in FIG. 1 cut in thenwell width direction around the center of the nwell finger.

FIG. 4 is an example DEPMOS finger end layout showing a finger endincluding a disclosed reduced doping finger edge region within the nwellfinger including over an active area/field dielectric boundary,according to an example embodiment.

FIG. 5 is a flow chart showing steps in an example method for forming anIC including a DEPMOS device including a disclosed reduced doping fingeredge region within the nwell finger including over an active area/fielddielectric boundary, according to an example embodiment.

FIG. 6 shows measured transient leakage data comparing a DEPMOS devicewith a known nwell finger design with a DEPMOS device having a disclosednwell finger design including a reduced doping finger edge region.

FIG. 7 shows measured DEPMOS low temperature peak transient leakage andRon Shift data for a DEPMOS device having a known nwell finger designcorresponding to the “no VTN” implant shown, and with a DEPMOS devicewith a disclosed nwell finger design including a reduced doping fingeredge region shown with a single or double VTN implant shown.

FIG. 8 shows measured DEPMOS Off leakage and BVdss characteristics for aDEPMOS device with a known nwell finger design and for a DEPMOS devicewith a disclosed nwell finger design including a reduced doping fingeredge region. The respective DEPMOS devices' BVdss and loffcharacteristics are shown have no significant difference.

DETAILED DESCRIPTION

Example embodiments are described with reference to the drawings,wherein like reference numerals are used to designate similar orequivalent elements. Illustrated ordering of acts or events should notbe considered as limiting, as some acts or events may occur in differentorder and/or concurrently with other acts or events. Furthermore, someillustrated acts or events may not be required to implement amethodology in accordance with this disclosure.

Also, the terms “coupled to” or “couples with” (and the like) as usedherein without further qualification are intended to describe either anindirect or direct electrical connection. Thus, if a first device“couples” to a second device, that connection can be through a directelectrical connection where there are only parasitics in the pathway, orthrough an indirect electrical connection via intervening itemsincluding other devices and connections. For indirect coupling, theintervening item generally does not modify the information of a signalbut may adjust its current level, voltage level, and/or power level.

FIG. 1 is top view depiction of an IC 150 including an example DEPMOSdevice 100 having a disclosed reduced doping finger edge region 160within the nwell fingers 120 ₁ and 120 ₂ including over an activearea/field dielectric boundary 140 a, according to an exampleembodiment. Although shown on IC 150, DEPMOS device 100 can also beembodied as a discrete die. The region shown to the right of the DEPMOSdevice 100 generally includes a plurality of other transistors, as wellas resistors and capacitors all configured together to provide a circuitfunction. DEPMOS device 100 is shown having a symmetric drain structure(has a symmetrical structure with respect to source and drain), althoughthis is not required as disclosed embodiments also apply to asymmetricdrain designs. Moreover, although the nwell fingers 120 ₁ and 120 ₂ areshown being rectangular in shape, the nwell fingers can have othershapes, such as having rounded corners.

The IC 150 comprises a substrate 105 having a doped surface layer 115thereon. The substrate 105 can be a bulk substrate material (e.g.,silicon) that provides the surface layer 115 too, or the surface layer115 can be an epitaxial layer on a substrate comprising a bulk substratematerial. The substrate 105 and/or surface layer 115 can comprisesilicon, silicon-germanium, or other semiconductor material. Thesubstrate 105 and surface layer 115 can both be n-type or p-type, withone specific embodiment being a p-type substrate and a p-type surfacelayer that is an epitaxial layer.

Although the DEPMOS device 100 is shown having two (2) nwell fingers 120₁ and 120 ₂, more generally, DEPMOS device 100 can have a single nwellfinger, or more than 2 nwell fingers. The nwell fingers 120 ₁, 120 ₂define an nwell length direction and a smaller nwell width direction.The nwell fingers 120 _(k), 120 ₂ have an nwell doping, and are formedwithin the doped surface layer 115, typically by ion implantation.

The nwell fingers 120 ₁, 120 ₂ are shown between pwells. Nwell finger120 ₁ is shown between the first pwell 125 a and the second pwell 125 b.Nwell finger 120 ₂ is shown between the second pwell 125 b and a thirdpwell 125 c. A p+ source (S) 126 is shown in the first pwell 125 a andp+drain (D) 136 is shown in the second pwell 125 b.

A gate stack is over a channel region 120 a of the nwell fingers 120 ₁,120 ₂ including over the nwell finger 120 ₁ between the S 126 and the D136. The gate stack includes a gate dielectric layer and a patternedgate electrode 130 on the gate dielectric layer (the gate dielectriclayer is not shown in FIG. 1, see FIG. 3 described below). The gateelectrode 130 can comprise polysilicon, or other gate electrodematerials such as a metal, and the gate dielectric can comprisedielectrics such as silicon oxide or silicon oxynitride.

A field dielectric layer 111 shown as a field oxide (FOX) layer is on aportion of the doped surface layer 115 defining active areas (where thefield dielectric layer 111 is lacking) including a first active area 140having a first active area/field dielectric boundary 140 a including afirst active area boundary along the nwell width direction (WD boundary)140 a 1. The field dielectric layer 111 can comprise a LOCOS oxidelayer, in which case there will be a birds beak region in the fielddielectric layer 111 transition region at the edge of the active areas.(See FIG. 2A described below). Alternatively, field dielectric layer 111can comprise shallow trench isolation (STI). In the case of STI, ifthere is a conventional added channel stop implant along the STI edge,the channel stop implant can benefit from a disclosed reduced dopingfinger edge region, such as being blocked off or being compensated withcounter doping similar to the LOCOS oxide edge case generally describedherein.

It is recognized herein upon thermal oxidation of silicon there is anincrease in n-type dopant concentration at the silicon surface exhibitedby n-type dopants such as arsenic and phosphorus referred to as“pile-up”. This enhancement in n-type dopant concentration at the Sisurface resulting from thermal oxidation can be a particularly importantfactor in LOCOS technology. The reduced doping finger edge region 160being at the nwell finger end close to first active area 140 reduces theotherwise excessive n-type doping present at the birds beak region (FOXtransition region) which has been found to reduce the field at theON-state with high back gate bias.

The nwell fingers 120 ₁, 120 ₂ each includes a reduced doping fingeredge region 160 over a portion of the WD boundary 140 a 1 on each end ofthe fingers. The reduced doping finger edge region 160 can comprise aregion counter doped with a p-type dopant, such as provided by an ionimplantation typically with boron. For example, a maximum concentrationof the p-type dopant in the reduced doping finger edge region 160 isgenerally between 1×10¹⁶cm⁻³to 1×10¹⁸ cm⁻³, which can be more or lessthan the nwell doping (so that reduced doping finger edge region 160 hasa reduced n-type level or becomes p-type). In one embodiment the borondose can be selected to be high enough to counter dope the nwell(s) inthe reduced doping finger edge region 160 to remain n-type to ensure ap-channel (when DEPMOS is ON) to nwell finger junction breakdown voltagethat is high enough for a given application. The reduced doping fingeredge region 160 can also comprise a region lacking direct nwell doping,such as by also masking this region (e.g. with photoresist) during thenwell implant. Due to lateral diffusion of the nwell, even withoutdirect (implanted) nwell doping, this region n-dopes the doping in thedoped surface layer 115, so that if the doped surface layer 115 isp-doped, this reduced doping finger edge region 160 generally becomesreduced p-type doping or reduced level n-type doping.

The reduced doping finger edge region 160 can extend a total of between1 μm and 4 μm in the nwell length direction. The reduced doping fingeredge region 160 is generally recessed from the edges of the nwellfingers 120 ₁, 120 ₂ as shown in FIG. 1 to help ensure low D136 to S 126leakage.

FIG. 2A is a cross sectional depiction of the DEPMOS device 100 shown inFIG. 1 cut in the nwell finger length direction around the activearea/field dielectric layer boundary along the nwell width direction.Here the field dielectric layer 111 comprises a LOCOS oxide layer 111′and the reduced doping finger edge region 160 can be seen to bepositioned in the nwell finger 120 ₁ birds beak region. A spacer 139 isshown on the edge of the gate electrode 130. The surface layer is shownbeing a p-epi surface layer 115′ that includes an n-buried layer (NBL)108 below the nwell finger 120 ₁. The surface of the nwell finger 120 ₁in the active area can be p-doped with a VTPHV surface layer that dopesthe active region to adjust the voltage threshold (Vt) of the buriedchannel PMOS.

FIG. 2B is a cross sectional depiction of the DEPMOS device 100 shown inFIG. 1 cut in the nwell width direction around the active area/fielddielectric boundary along the nwell width direction. The first andsecond pwells 125 a, 125 b are shown. The nwell finger 120 ₁ under theLOCOS oxide layer 111′ is shown having a reduced doping finger edgeregion 160, such as by counter doping the nwell doping with a p-typeimplant.

FIG. 3 is a depiction of the DEPMOS device 100 shown in FIG. 1 cut inthe nwell width direction around the center of the nwell finger to showthe gate dielectric 131 under the gate electrode 130. In the embodimentshown the substrate 105 is a p-type substrate. Various surface contactsare shown through an interlevel dielectric (ILD) shown as ILD 165,including contact 126 a to the S, contact 136 a to the D, and contacts137 a and 138 a to the back gate (BG). Since as disclosed abovedisclosed reduced doping finger edge regions 160 are provided at thefinger edges along the edges in the finger length direction, there is noreduced doping finger edge region 160 provided in the center portion ofthe DEPMOS device 100 as shown in FIG. 3.

The DEPMOS device in FIG. 3 is shown including a first additional nwellfingers 120 ₃ beyond the first pwell 125 a opposite the nwell finger 120₁ and a second additional nwell finger 120 ₄ beyond the second pwell 125b opposite the nwell finger 120 ₁ The additional nwell fingers 120 ₃ and120 ₄ together with the NBL 108 shown which functions as an n-typesinker along with the deep NBL (DNBL) 109 which form an n-type ‘tank’that through reverse biasing during DEPMOS device operation enables theS 126 and D 136 of the DEPMOS device to be junction isolated from othercomponents on the die, such as from other transistors. However, if thesubstrate 105 is instead ntype, there is generally no need for the DNBL109 and NBL 108 to provide isolation for the S 126 and the D 136 of theDEPMOS device.

FIG. 4 is an example DEPMOS finger end layout showing a finger endincluding a disclosed reduced doping finger edge region 160′ within thenwell finger 120 ₁ including over an active area/field dielectricboundary 140 a, according to an example embodiment. Here the disclosedreduced doping finger edge region 160′ lacks the nwell doping, such asby masking the reduced doping finger edge region 160′ during nwellimplant. Such a dopant ‘hole’ in the nwell finger 120 ₁ also reduces then-type doping pile up under the birds beak region that can cause impactionization at ON-state with high back gate bias levels causing increasedtransient leakage and parametric shifts as described above.

FIG. 5 is a flow chart showing steps in an example method 500 forforming an IC having a DEPMOS device including a disclosed reduceddoping finger edge region within the nwell finger including over anactive area/field dielectric boundary, according to an exampleembodiment. Step 501 comprises providing a substrate 105 having a dopedsurface layer 115 thereon. Step 502 comprises forming at least one nwellfinger 120 defining a length direction and a width direction having annwell doping within the surface layer 115 including a channel region 120a for the device. A typical nwell implant dose is of about 1×10¹³ cm⁻²to provide an approximate nwell doping level 1×10¹⁶ to 1×10¹⁷ cm⁻³. Inone embodiment the nwell masking level includes a masking material(e.g., photoresist) over the region corresponding to the reduced dopingfinger edge region 160 so that an ion implant used to form the nwellfinger 120 is blocked.

Step 503 comprises the counterdoping embodiment for forming the reduceddoping finger edge region 160. Step 504 comprise implanting boron in thereduced doping finger edge region 160 at a dose that can be between1×10¹² cm⁻² and 1×10¹³ cm⁻², and at an energy that can be between 90 KeVand 200 KeV. As a result, the reduced doping finger edge region 160 canbe either lightly n-doped or lightly p-doped. Although step 503 isdescribed being before forming the gate stack (step 506 describedbelow), step 503 can also be performed after forming the gate stack(step 506).

Step 504 comprises forming a first pwell 125 a on one side of the nwellfinger 120 and a second pwell 125 b on an opposite side of the nwellfinger 120. Boron ion implantation can be used to form the pwells. Step505 comprises forming a field dielectric layer 111 on a portion of thedoped surface layer 115 defining active area boundaries including afirst active area 140 having a first active area boundary 140 aincluding a first active area boundary along the width direction (WDboundary) 140 a 1 that has the channel region 120 a therein. As notedabove, the field dielectric layer 111 can comprises a LOCOS oxide or STIoxide.

Step 506 comprises forming a gate stack between over the channel region120 a including a gate dielectric layer 131 and a patterned gateelectrode 130 on the gate dielectric layer 131. Step 507 comprisesforming a p+ source in the first pwell 125 a and a p+ D in the secondpwell 125 b.

As noted above, in one embodiment the field oxide layer comprises aLOCOS oxide layer and a p-type layer is selectively implanted at thefinger ends inside the nwell finger close to the active region edge.This added p-type layer counter dopes the excessive n-type doping in thenwell in the birds beak region reducing the field for the DEPMOS devicewhen operating in the ON-state with a high back gate bias level.Disclosed reduced doping finger edge regions thus prevent DEPMOS deviceON-state leakage from the finger end near the active area edge withminimum impact to the intrinsic device characteristics (see Examplessection below). Disclosed reduced doping finger edge regions do notrequire any additional masks, as the nwell implant blocking embodimentuses a modified nwell mask, and the implanted counterdoped embodimentcan generally utilize an existing implant in the process flow, such asan n-channel threshold adjust implant in a CMOS process flow or in aBiCMOS process flow.

EXAMPLES

Disclosed embodiments are further illustrated by the following specificExamples, which should not be construed as limiting the scope or contentof this Disclosure in any way.

FIG. 6 shows measured transient leakage data comparing a DEPMOS devicewith a known nwell finger design (no reduced doping finger edge region160) with a disclosed DEPMOS device with a disclosed nwell finger designincluding a reduced doping finger edge region 160. The back gate voltage(V_(h)) was 20V and the source voltage V_(S) was floating. The uppergraphs are the absolute leakage values and the lower graphs are theleakage contour vs. V_(G) and V_(D).

FIG. 7 shows measured DEPMOS low temperature peak transient leakage andthe Ron Shift for a DEPMOS device having a known nwell finger design (noreduced doping finger edge region 160) corresponding to the “no VTN”shown, and with a DEPMOS device with a disclosed nwell finger designincluding a reduced doping finger edge region 160 shown with a single ordouble VTN implant shown. The VTN implant was a chain of boron implantswith a deepest implant in the chain being at a dose of 5×10¹² cm⁻² at165 Key. The DEPMOS On-state peak leakage is shown at −40° C.,VG/VB=−22V/22V, V_(S)=Open, V_(D) sweeps: −22V to 22V on the top, andthe Ron shift after a stress of VG/VB=22V/22V, V_(S)=Open, V_(D) sweeps:−22V to 22V at the bottom.

FIG. 8 shows measured DEPMOS Off leakage and BVdss characteristics for aDEPMOS device with a known nwell finger design and for a DEPMOS devicewith a disclosed nwell finger design including a reduced doping fingeredge region. The respective DEPMOS devices' BVdss and loffcharacteristics can be seen to have no significant difference.

Disclosed embodiments can be used to form semiconductor die includingdiscrete or IC die that may be integrated into a variety of assemblyflows to form a variety of different devices and related products. Thesemiconductor die may include various elements therein and/or layersthereon, including barrier layers, dielectric layers, device structures,active elements and passive elements including source regions, drainregions, bit lines, bases, emitters, collectors, conductive lines,conductive vias, etc. Moreover, the semiconductor die can be formed froma variety of processes including bipolar, Insulated Gate BipolarTransistor (IGBT), CMOS, BiCMOS and MEMS.

Those skilled in the art to which this disclosure relates willappreciate that many other embodiments and variations of embodiments arepossible within the scope of the claimed invention, and furtheradditions, deletions, substitutions and modifications may be made to thedescribed embodiments without departing from the scope of thisdisclosure. For example, the DEPMOS device can be in a race tracklayout.

1. A method of fabricating an integrated circuit (IC) having a p-channeldrain extended metal oxide semiconductor (DEPMOS) device, comprising:providing a substrate having a doped surface layer thereon; forming atleast one nwell finger defining a nwell length direction and an nwellwidth direction having nwell doping within said doped surface layerincluding a channel region; forming a first pwell on one side of saidnwell finger and a second pwell on an opposite side of said nwellfinger; forming a field dielectric layer on a portion of said dopedsurface layer defining active area boundaries including a first activearea having a first active area boundary including a first active areaboundary along said width direction (WD boundary) that has said channelregion therein; forming a gate stack over said channel region includinga gate dielectric layer and a patterned gate electrode on said gatedielectric layer, and forming a p+ source in said first pwell and a p+drain in said second pwell, wherein said method includes reduced dopingfinger edge region processing which provides a reduced doping fingeredge region including over a portion of said WD boundary within saidnwell finger.
 2. The method of claim 1, wherein said field dielectriclayer is formed by a Local Oxidation of Silicon (LOCOS) process.
 3. Themethod of claim 1, wherein said field dielectric layer is formed by ashallow trench isolation (STI) process including an etch and then asilicon oxide fill.
 4. The method of claim 1, wherein said reduceddoping finger edge region processing comprises ion implantation of ap-type dopant to form said reduced doping finger edge region by counterdoping.
 5. The method of claim 4, wherein said ion implantationcomprises a boron dose between 1×10¹² and 1×10¹³ cm⁻² and an energybetween 90 KeV and 200 KeV.
 6. The method of claim 1, wherein saidwherein said reduced doping finger edge region processing uses a maskingmaterial in said reduced doping finger edge region during an n-type ionimplant used to form said nwell finger.
 7. The method of claim 1,wherein said at least one nwell finger comprises a plurality of saidnwell fingers.
 8. The method of claim 1, wherein said doped surfacelayer is doped ptype, wherein said forming said at least one nwellfinger further comprises forming a first additional nwell beyond saidfirst pwell opposite said nwell finger and a second additional nwellbeyond said second pwell opposite said nwell finger, and wherein saidmethod further comprising forming an n-buried layer (NBL) and an n-typesinker in said substrate so that said first and said second additionalnwells together with said NBL and said n-type sinker form an isolationtank.
 9. A p-channel drain extended metal oxide semiconductor (DEPMOS)device, comprising: a substrate having a doped surface layer thereon; atleast one nwell finger defining a nwell length direction and an nwellwidth direction having nwell doping formed within said doped surfacelayer; a first pwell on one side of said nwell finger including a p+source therein and a second pwell on an opposite side of said nwellfinger including a p+ drain; a gate stack defining a channel region ofsaid nwell finger between said source and said drain including a gatedielectric layer and a patterned gate electrode on said gate dielectriclayer; a field dielectric layer on a portion of said doped surface layerdefining active area boundaries including a first active area having afirst active area boundary including a first active area boundary alongsaid width direction (WD boundary), wherein said nwell finger includes areduced doping finger edge region over a portion of said WD boundary.10. The DEPMOS of claim 9, wherein said field dielectric layer comprisesa Local Oxidation of Silicon (LOCOS) oxide.
 11. The DEPMOS of claim 9,wherein said reduced doping finger edge region comprises a regioncounterdoped with a p-type dopant.
 12. The DEPMOS of claim 9, whereinsaid reduced doping finger edge region comprises a region lacking saidnwell doping.
 13. The DEPMOS of claim 9, wherein said reduced dopingfinger edge region extends a total of between 1 μm and 4 μm in saidnwell length direction, and is recessed from an edge of said nwellfinger in said nwell width direction.
 14. The DEPMOS of claim 9, whereinsaid at least one nwell finger comprises a plurality of said nwellfingers.
 15. The DEPMOS of claim 9, wherein said DEPMOS device has asymmetric drain structure.
 16. An integrated circuit (IC) comprising asubstrate including a doped surface layer thereon having a p-channeldrain extended metal oxide semiconductor (DEPMOS) device formed in saiddoped surface layer, said DEPMOS device comprising: at least one nwellfinger defining a nwell length direction and an nwell width directionhaving nwell doping formed within said doped surface layer; a firstpwell on one side of said nwell finger including a p+ source therein anda second pwell on an opposite side of said nwell finger including a p+drain; a gate stack defining a channel region of said nwell fingerbetween said source and said drain including a gate dielectric layer anda patterned gate electrode on said gate dielectric layer, and a fielddielectric layer on a portion of said doped surface layer definingactive area boundaries including a first active area having a firstactive area boundary including a first active area boundary along saidwidth direction (WD boundary), wherein said nwell finger includes areduced doping finger edge region over a portion of said WD boundary.17. The IC of claim 16, wherein said field dielectric layer comprises aLocal Oxidation of Silicon (LOCOS) oxide.
 18. The IC of claim 16,wherein said reduced doping finger edge region comprises a regioncounterdoped with a p-type dopant.
 19. The IC of claim 16, wherein saidreduced doping finger edge region comprises a region lacking said nwelldoping.